1. Field of the Invention
The present invention pertains to drum freezers, and in particular to such freezers cooled by a liquid cryogen or other heat transfer fluid.
2. Description of the Related Art
There are many advantages to cryogenic freezing of food products which have come to light in the past several decades, and in a number of instances, carbon dioxide is the cryogen of choice for efficient and economical cryogenic freezing applications. Cryogenic carbon dioxide food freezers often utilize liquid carbon dioxide under pressure sufficient to maintain it in the liquid state and supply it to spray nozzles through which it is injected into the interior of a thermally insulated enclosure wherein the food products to be frozen are delivered to a freezing region, as by being transported on an endless conveyor or the like. In a CO.sub.2 food freezer, the low temperatures which can be achieved by the creation of solid CO.sub.2, can create a tendency for liquid CO.sub.2 in the lines leading to the spray nozzles to freeze, particularly at times when there is no flow or only very low flow. Gassing systems have been devised and utilized to clear the lines of liquid CO.sub.2 at certain times to prevent such freezing.
It is well known in the art to use drum flakers in the chemical process industry and to a lesser extent, in the food manufacturing industry. Drums have also been used for cooling or freezing various products, including protein materials, as will be discussed herein. Typical cooling media for drum coolers or freezers are water, chilled water, refrigerated brine solutions, fluorocarbon refrigerants, such as chloro-fluorocarbons or chloro-fluoro-hydrocarbons (CFC's or HCFC's), and mechanically refrigerated ammonia. The use of liquid nitrogen as a heat transfer media for cooling drums has never been successfully commercialized. Drum freezers are very efficient for cooling or freezing, (i.e., solidifying) liquid or semi-soft foods or chemical products as compared to other, conventional methods such as trickle chillers, plate or tray freezers, or blast freezer rooms.
For example, a variety of products, such as hamburger meat and other protein materials are prepared as a viscous paste and applied to the surface of a refrigerated drum, which is cooled to a sufficiently low temperature so as to cause freezing of the protein material into a continuous sheet. For example, U.S. Pat. Nos. 4,098,095; 4,337,627; and 4,349,575 disclose cooling drums for a freezing viscous paste of hamburger meat and the like protein materials.
In U.S. Pat. No. 4,098,095, a fluid refrigerant such as FREON or ammonia is circulated through the interior of the drum and the drum is rotated, in synchronism with a belt surrounding a portion of the drum surface. A viscous paste is applied to a portion of the cooling drum surface located upstream of the belt. The paste material is spread uniformly across the drum surface, and the paste material adheres to the drum surface with freezing occurring upon contact with the drum surface, or shortly thereafter. As the viscous paste travels with the drum surface, interior portions of the paste blanket become frozen and eventually the surface of the paste blanket remote from the drum surface also freezes.
The paste blanket rotates with the drum to contact the belt. The belt presses the blanket against the cooling drum surface to aid in the uniform distribution of paste material across the drum surface, thereby providing a blanket of consistent, uniform thickness. The blanket emerging from the downstream end of the belt is drawn away from the surface of the freezing drum by a knife blade, and is passed to processing equipment located downstream of the refrigeration apparatus.
U.S. Pat. No. 4,337,627 discloses a similar arrangement of a cooling drum and a conveyor belt surrounding a portion of the drum surface. The temperature of the viscous paste and of the cooling drum are maintained such that the viscous paste sticks to the drum surface shortly after contact therewith.
U.S. Pat. No. 4,349,575, also by the same inventor, discloses a refrigeration drum with a conveyor belt surrounding a major portion of the drum surface. The viscous paste is fed into a nip formed between the upstream end of the conveyor belt and the drum surface, the conveyor belt pressing the viscous paste into contact with the drum surface and maintaining such pressure as the paste travels with the rotating drum. The conveyor belt holds the viscous paste in contact with the drum before and after freezing of the paste is completed.
European Patent Application No. 332,287 discloses a drum freezer for egg products. Liquid nitrogen is circulated in the drum interior while liquid egg products are deposited, drop by drop on the drum surface. Upon contact with the drum surface, the drops freeze and are collected for transport or further processing. A conveyor belt surrounding the drum surface is not employed.
There is a continuing demand for an improved drum freezer apparatus resulting in increased economy and increased throughput capability.